Over the last decade microfluidic systems have developed into valuable instrumental platforms for performing high throughput chemistry and biology (deMello (2006) Nature, 442: 394-402). The ability to controllably merge droplets within segmented flow systems is of high importance when performing complex chemical or biological analyses (Shestopalov et al. (2004) Lab Chip, 4: 316-321). Unfortunately, the controlled merging of multiple droplets in a sequential fashion is not straightforward. Although the emulsions produced in microfluidic systems are thermodynamically metastable, the process of merging has not proven to be predictable, due to subtle variations in interfacial tension, surface topography of microchannels, and fluidic properties such as of droplet size, viscosity, and velocity (see, e.g., Fuerstman et al. (2007) Science, 315: 828-832).
Droplet merging is important or essential in many applications including sequential reactions (Kim et al. (2006) Anal. Chem., 78(23): 8011-8019), multiple step manipulation of cells (He et al. (2005) Anal. Chem., 77(6): 1539-1544), high-throughput bioassays (Srisa-Art et al. (2007) Anal. Chem., 79: 6682-6689), and the like. Additionally, the ability to merge and split droplets or bubbles in a high throughput manner cab impact the use of bubble logic systems for exchanging chemical and electronic information (Prakash and Gershenfeld (2007) Science, 315(5813): 832-835).
In typical droplet merging processes, relatively large time and spatial scales are involved. For example, timescales may range from the sub-microsecond regime for some chemical reactions to many hours and even days for cell-based assays. Similarly, large spatial scales also exist, for example, between the droplets to be merged and between the droplets and the component interfaces that interact to drive the merging process.
Several techniques have been developed to merge droplets. These are either active and involve components such as electric fields (Priest et al. (2006) Appl. Phys. Lett., 89: 134101:1-134101:3; Ahn et al. (2006) Appl. Phys. Lett., 88: 264105), or passive and utilize the surface properties (Fidalgo et al. (2007) Lab Chip, 7(8): 984-986) or structure (Tan et al. (2004) Lab Chip, 4(4): 292-298) of the fluidic conduit.